Cellulitis is an infection in the soft tissues under the skin and can occur in any area of the body but is more common in the hind limbs. Often there is a point of entry within the skin for bacteria to gain access to body, and this can be a small wound, an abrasion, or a fine crack in the skin. The unusual aspect of peri-tarsal cellulitis (PTC), however, is that the infection presents as a similar swelling around the hock in all horses affected by the condition and an entry wound for bacteria is seldom identified.
One theory put forward to explain why the hock is predisposed to this condition involves the pattern of the blood vessels that supply the hock and how this predisposes the area to bacteria becoming lodged in the soft tissues, however, this has not been proven.
Horses with PTC present with a sudden-onset severe lameness that, on initial presentation, is often confused with a fracture or an infection of the hock joint because of the swelling localised around the hock joint; it is painful to touch or palpate, with no visible signs of an injury present. Within hours to days, the swelling spreads up and down the leg, resulting in a grossly enlarged hind leg with the horse often refusing to move because of the pain.
A clinical examination should lead the veterinarian towards a diagnosis of an infection, as the rectal body temperature is usually elevated (over 39°C), higher than would be expected with pain associated with a limb fracture. Limb fractures can cause a mild increase in rectal temperature due to pain, but seldom is this over 39°C.
X-RAYS & TESTS
The sudden onset and the degree of pain will prompt some practitioners to X-ray the limb to confirm there is no fracture present. In uncomplicated cases, no fractures will be detected but a large degree of soft tissue swelling will be visible on the X-ray. A blood test will show marked evidence of an infection, however, in most ambulatory situations, results cannot be obtained for several hours after the initial presentation, so treatment is started before a definitive diagnosis can be made.
In some very early cases, the blood inflammatory/infection markers have not had enough time to rise, giving a false negative reading on the initial blood, however, a follow-up blood test should always be performed if the vet believes the leg is infected and the initial blood is inconclusive. The second blood will show marked increases in inflammatory/infection markers, the white cells, the fibrinogen, and the serum amyloid A (SAA) if it is PTC.
The more complex question initially is whether the infection is only in the soft tissues around the joint, or whether it involves the hock joint itself. An ultrasound examination can be helpful to assess the amount and clarity of joint fluid in the hock, however, it is very painful for the horse to undergo this procedure, due to the pressure needed to be applied to the probe on the site to get a good image, requiring most horses to be sedated.